Rolls for disposing at entry side or exit side of quenching zone of continuous annealing furnace and quenching zone unit using rolls

ABSTRACT

A roll that can be disposed before and/or after a quenching zone of a continuous annealing furnace is provided. The roll has a predetermined profile satisfying Lc≧0.7×W min ; R=−0.1×10 −3  to +0.2×10 −3 ; and TR≧20, wherein Lc represents the length (mm) of a flat portion in the center of the roll, W min  represents the minimum width (mm) of a steel strip, R represents the inclination of tapered portions disposed at two sides of the roll, and TR represents the radius of curvature (m) of boundaries between the flat portion and the tapered portions. The roll can be used as a hearth roll and/or a bridle roll disposed at the entry side and/or the exit side of the quenching zone.

BACKGROUND OF THE INVENTION

[0001] 1. Field of Invention

[0002] The invention relates to rolls disposed before and/or after aquenching zone of a continuous annealing furnace for continuouslyheat-treating strip and to quenching zone unit including the rolls.

[0003] 2. Description of Related Art

[0004] As the size of automotive vehicles has increased in recent years,the width of steel strips has also increased. Moreover, from the pointof view of preventing global warming, high-strength steel plates areincreasingly employed to achieve a vehicular weight reduction bydown-gauging the steel strips.

[0005] High-strength steel strips having increased widths and reducedthicknesses are being produced using continuous annealing furnaces.Continuous annealing furnaces are now required to treat a steel striphaving an increased width ranging from 600 mm to 1850 mm and to 2100 mmin some cases.

[0006] Moreover, the annealing temperature of steel strips is furtherelevated, and, consequently, the steel strip passing through the furnaceis further softened, resulting in increased amounts of defectiveproducts. For quality control, more precise control of the rapid coolingoperation after annealing is required.

[0007] Because of these reasons, conventional processes are no longersufficient for achieving stable operations of continuous annealingfurnaces.

[0008] As shown in FIG. 4, an upright continuous annealing furnace forannealing steel strip comprises a heating zone 2 for heating a steelstrip to a predetermined temperature to perform annealing, a soakingzone 3, and a cooling zone for cooling the high-temperature material,i.e., the steel strip 1, to a room temperature.

[0009] The cooling zone normally comprises a plurality of furnace zones,namely, a quenching zone 4 (or “primary cooling zone”) for rapid coolingthe high-temperature steel strip, an over-aging zone 5, and a secondarycooling zone 6.

[0010] Before and after the quenching zone 4, i.e., at the entry sideand the exit side of the quenching zone 4, hearth rolls or bridle rollsfor feeding the steel strip 1 are provided. Moreover, bridle roll units8 for preventing fluttering of the steel strip 1 inside the quench zone4 are provided in many cases.

[0011] Herein, the term “quenching zone unit” includes the quenchingzone 4 and the rolls, such as the bridle roll unit 8 disposed before andafter the quenching zone 4.

[0012] Although the upright continuous annealing furnace shown in FIG. 4includes the over-aging zone 5 and the secondary cooling zone 6, theover-aging zone 5 and the secondary cooling zone 6 may be omitted whenapplied to, for example, a molten metal plating line.

[0013] By employing the process of quenching a high-temperature metalstrip in a quenching zone, the quality of the steel strip can beadequately controlled and the resulting products have both sufficientformability and sufficient strength. An exemplary steel strip beingsteel plates for vehicular bodies having a baking hardening property.

[0014] As processes for quenching the steel strips, a gas jet coolingprocess comprising cooling the atmospheric gas in the quenching zoneusing a heat exchanger, circulating the gas, and blowing cooled gas jetstreams at high speeds to the steel strips; a roll cooling processcomprising cooling rolls by placing cooling media into the rolls andpressing the rolls against a steel strip to quench the steel strip; awater quenching process using water as a cooling medium; a mist coolingprocess, and the like, are known.

[0015] Among these processes, the gas jet cooling process advantageouslyprovides steel strips having satisfactory appearance and shapes aftercooling. Moreover, the cost for the cooling equipment is relatively low.Thus, a high-speed gas jet cooling process, in which a temperature rangeof 300° C. or more is quenched using a quenching zone including gas jetcooling equipment having a heat transfer coefficient of 170 W/(m²·° C.)or more per surface, is now being performed. Herein, the phrase “atemperature range of 300° C. or more is rapidly quenched” means that thetemperature of the steel strip that is quenched is 300° C. or more atthe entry side of the quenching zone.

[0016] However, in the high-speed gas jet cooling process, the coolingair hitting the steel strip surfaces reaches connection sections, forexample, the bridle roll unit, disposed between the heating zone and thecooling zone, thereby over-cooling the edge portions of the hearth rollsor the bridle rolls installed in the connection sections and generatinglarge thermal crowns in the centers of these rolls. Consequently, thesteel strip suffers from buckling in the width direction.

[0017] The following publications disclose means for solving thisproblem. Japanese Unexamined Patent Application Publication No. 56-65942discloses that fluttering of steel strip is reduced by providinginner-furnace bridle rolls at the quenching zone entry side andincreasing the tension of steel strip at a gas jet nozzle unit. However,operational experiences demonstrate that the buckling of steel strip atthe inner-furnace bridle rolls disposed at the quenching zone entry sidecannot be completely prevented.

[0018] Japanese Unexamined Patent Application Publication No. 60-40463discloses a seal for preventing gas leakage from the connecting portion.However, when the seal for preventing gas leakage is applied to ahigh-speed gas jet cooling unit, the cooling gas hitting the steel stripsurfaces leaks from the connecting units provided before and after thequenching zone, thereby generating a temperature distribution in therolls disposed in the bridle roll units installed at the entry side andexit side of the quenching zone and resulting in the buckling of thesteel strip. Accordingly, the bridle roll units require additional meansfor solving this problem. Otherwise, the edge portions of the rollsinside the bridle roll unit are excessively cooled, large thermal crownsare developed in the centers of these rolls, and buckling occurs in thewidth direction of the steel strip.

[0019] Japanese Unexamined Patent Application Publication No. 6-93347discloses that kinetic energy is reduced by disposing a sealingapparatus at an upper portion of a gas jet chamber of a quenching zoneand injecting, from the sealing apparatus, a stream which flows in adirection opposing the stream at the surface of the steel strip.However, the sealing apparatus requires installation of a counter-streaminjection apparatus and a seal roll, resulting in increased costs.Moreover, the operation thereof is complicated.

[0020] Japanese Unexamined Patent Application Publication No. 9-268324discloses that in a quenching zone, the angle of a roll crown isadjusted so as to control the buckling threshold tension to be largerthan the tension of the steel strip. However, in this continuous heattreating process, the rolls having a roll angle of the disclosed rangedo not come into satisfactory contact with the steel strip, resulting inslipping between the rolls and the steel strip.

SUMMARY OF THE INVENTION

[0021] Accordingly, objects of the invention are to solve theabove-described problems and to achieve a high-speed gas jet coolingprocess without causing defects, such as buckling and meandering, evenwhen rapid cooling is performed in a temperature range of 300° C. ormore and at a quenching zone having a jet cooling unit having a heattransfer coefficient per surface of 170 W/(m²·° C.). The invention canachieve a reliable operation of a continuous annealing furnace byallowing a steel strip of a reduced gauge and an increased width tostably pass through the line. The invention also can solve problems suchas decrease in yield, decrease in line speed, and shutdown.

[0022] In order to achieve these goals, an exemplary embodiment of theinvention provides a roll to be disposed before or after a quenchingzone of a continuous annealing furnace, satisfying the followingrelationships:

[0023] Lc≧0.7×W_(min);

[0024] R=−0.1×10⁻³to +0.2×10⁻³; and

[0025] TR≧20

[0026] wherein Lc represents the length (mm) of a flat portion in thecenter of the roll, W_(min) represents the minimum width (mm) of a steelstrip, R represents the inclination of tapered portions disposed at thetwo sides of the roll, and TR represents radius of curvature (m) of theboundaries between the flat portion and the tapered portions.

[0027] Another exemplary embodiment of the invention provides aquenching zone unit of a continuous annealing furnace, comprising atleast one hearth roll and/or at least one bridle roll disposed at theentry side and/or the exit side of a quenching zone. The above-describedroll comprises the hearth roll and/or bridle roll.

[0028] Another exemplary embodiment of the invention provides aquenching zone unit of a continuous annealing furnace, including atleast one bridle roll unit having a plurality of rolls. The at least onebridle roll unit is provided at the entry side and/or the exit side of aquenching zone. The above-described exemplary roll comprises each ofthese rolls.

[0029] Preferably, the roll closest to the quenching zone is a flat rollsatisfying the relationships (i) R=0 and (ii) TR=∞.

[0030] Preferably, at least one pair of seal rolls is disposed at theentry side and/or the exit side of the quenching zone.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 illustrates the vicinity of a quenching zone according toan exemplary embodiment of the invention;

[0032]FIG. 2 illustrates the vicinity of a quenching zone according toanother exemplary embodiment of the invention;

[0033]FIG. 3 illustrates the vicinity of a quenching zone of ahorizontal continuous annealing furnace;

[0034]FIG. 4 illustrates the structure of a continuous annealingfurnace;

[0035]FIG. 5 shows an exemplary embodiment of a roll according to theinvention;

[0036]FIG. 6 is a graph showing the relationship between a length Lc ofthe flat portion of a roll and a trouble ratio;

[0037]FIG. 7 is a graph showing the relationship between an inclinationR of the tapered portion of a roll and a trouble ratio;

[0038]FIG. 8 is a graph showing the relationship between the length Lcof the flat portion of the roll at the entry side and the exit side ofthe quenching zone and problems regarding feeding;

[0039]FIG. 9 is a graph showing the relationship between the inclinationR of the tapered portion at the entry side and the exit side of thequench zone and problems regarding feeding;

[0040]FIG. 10 is a graph comparing yield reduction rates of aconventional process and the process of the invention; and

[0041]FIG. 11 is a graph comparing operational efficiency reductionrates of the conventional process and the process of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0042] Preferred embodiments of the invention will be described withreference to the drawings.

[0043] Although the rolls of the invention can be preferably applied tothe continuous annealing furnace shown in FIG. 4, the invention is notlimited to this application. Rather, rolls according to the inventioncan be applied to a wide variety of continuous annealing furnaces havingquenching zones.

[0044] The structures of a typical quenching zone and quenching zoneunits disposed before and after the quenching zone according to theinvention will be described with reference to FIG. 1.

[0045] In FIG. 1, a quenching zone 4 includes a gas jet coolingapparatus 12 and performs quenching of a steel strip 1 fed into thequenching zone 4. In order to give a target tension to the steel strip 1and to prevent fluttering of the steel strip 1 inside the quenching zone4, bridle roll units 8 are disposed before and after the quenching zone4. Seal rolls 11 are disposed inside the respective bridle roll units 8to prevent the cooling gas ejected in the gas jet cooling apparatus 12from reaching entering the bridle roll units 8. Also, a heater 7 isprovided in each of the bridle roll units 8 to prevent temperature dropsinside the bridle roll units 8 and to maintain the temperature at apredetermined temperature. A plurality of hearth rolls 9 and bridlerolls 10 are provided inside each of the bridle roll units 8.

[0046] In FIG. 1, two of the hearth rolls 9 and three of the bridlerolls 10 are installed inside the bridle roll unit 8 located near theentry side of the quenching zone 4 and one hearth roll 9 and three ofthe bridle rolls 10 are installed inside the bridle roll unit 8 locatednear the exit side of the quenching zone.

[0047] The invention is, however, not limited to the particularconfiguration shown in FIG. 1. Other configurations can also be used aslong as the steel strip 1 is provided with a target tension. Forexample, as shown in FIG. 2, the hearth roll 9 may also function as thebridle roll and the bridle roll unit 8 may be provided with three bridlerolls 10.

[0048] Furthermore, the invention can be applied to a horizontalcontinuous annealing furnace as shown in FIG. 3.

[0049] Embodiments of the invention can optimize the thermal crowns ofthe hearth rolls 9 and the bridle rolls 10 installed inside the bridleroll units 8 and prevent the steel strip 1 from slipping, buckling, andmeandering by a desired tension applied to the steel strip.

[0050] The rolls installed after and before the quenching zone accordingto the invention can be applied to both the hearth rolls and the bridlerolls.

[0051] Next, a roll profile for achieving stable strip feeding withoutslippage of the steel strip will be described in detail.

[0052] As illustrated in FIG. 5, an exemplary roll 20 according to theinvention can be disposed after or before the quenching zone. The roll20 comprises a substantially flat portion 22 having a length Lc andtapered portions 24 having an inclination R. The flat portion 22 issandwiched by the tapered portions 24, and the roll 20 is therebysymmetrical. the roll crown has a convex shape. When the inclination Ris negative, the roll crown has a concave shape. The inclination R isdefined as the ratio of the length L of the tapered portion to the valueC which equals half the difference in outer diameters between thebeginning of the tapered portion and the end of the tapered portion,i.e., R=C/L.

[0053] According to the invention, the flat portion 22 does notnecessarily need to be exactly flat. For example, the flat portion mayhave a gently curved surface having a radius of curvature of 100 m ormore.

[0054] In order to specify preferred ranges of the invention, therelationship between the length Lc of the flat portion of the roll andthe ratio of problems caused by slippage between the roll surface andthe steel strip was examined. The results are shown in FIG. 6. In thisexamination, conditions such as the inclination R of the roll taperedportion and the radius of curvature at the boundary between the flatportion and the tapered portions were kept within preferred conditionsaccording to the invention. Regarding cooling conditions, a temperaturerange of 300° C. or more was quenched using a quenching zone includinggas jet cooling equipment having a capacity of 170 W/(m²·° C.) or more,reduced to a heat transfer coefficient per steel strip surface. Thetrouble ratio in the graph is normalized by the average of conventionaloperational data.

[0055] The examination shows that the preferred length of the flatportion (Lc) of the roll relative to the minimum strip width W_(min) ofsteel strip fed to the flat portion satisfies condition (1):

[0056] Lc≧0.7×W_(min)

[0057] Next, the relationship between the inclination R of the taperedportion and the problem ratio caused by the slippage between the steelstrip and the roll surface was examined. The results are shown in FIG.7. Other conditions such as the length Lc of the flat portion of theroll, the radius of curvature of the boundary between the flat portionand the tapered portions, etc. were within the preferred conditions ofthe invention, and the cooling conditions were the same as those in theexamination regarding FIG. 6. The trouble ratio is normalized by theaverage value of conventional operational data.

[0058] Accordingly, the range of the inclination R of the taperedportion preferably satisfies condition (2):

[0059] R=−0.1×10⁻³to +0.2×10⁻³

[0060] Furthermore, the boundary between the roll flat portion and thetapered portions is preferably smooth and round without edges in orderto prevent slipping and buckling of the steel strips. To smooth theboundary between the flat portion and the tapered portions of the roll,the radius of the curvature TR at the boundary is preferably 20 m ormore. In other words, the curvature TR satisfies condition (3):

[0061] TR≧20

[0062] Next, based on the above-described parameters, the conditionswhich prevent buckling and meandering of the steel strip wereinvestigated. The results are shown in FIG. 8. In FIG. 8, the horizontalaxis indicates the entry side and the exit side of the quenching zoneand the vertical axis indicates the length Lc of the roll flat portion.R and TR were set within the ranges of the invention. The graph showswhether buckling and/or meandering occurred in the steel strip fedtherein. The cooling conditions were the same as those in theexamination regarding FIG. 6. In the graph, circles (◯) indicate thatthe steel strip had no defect and squares (□) indicate that buckling wasobserved in the steel strip. The speed of the strip line was set to anormal speed (100 to 300 m/min). No meandering was observed in theexamination regarding FIG. 8.

[0063]FIG. 8 also demonstrates that condition (1):

[0064] Lc≧0.7×W_(min)

[0065] is preferable.

[0066]FIG. 9 is a graph indicating the generation of buckling andmeandering when a steel strip was fed. The horizontal axis in the graphindicate the entry side and the exit side of the quenching zone. Thevertical axis in the graph indicate the inclination R of the taperedportion. The conditions such as Lc and TR were set within the preferredrange of the invention. The cooling conditions were the same as those inthe examination regarding FIG. 6. In the graph, circles (◯) indicate thesteel strip had no defect, triangles (Δ) indicate that meandering wasobserved, and squares (□) indicate that buckling was observed. The speedof the strip line was set to a normal speed (100 to 300 m/min).

[0067]FIG. 9 also demonstrates that condition (2):

[0068] R=−0.1×10⁻³ to +0.2×10⁻³

[0069] is preferable. This range is actually wider than the preferredrange for preventing problems caused by slippage as shown in FIG. 7.

[0070] In order to prevent the steel strip from vibrating inside thequenching zone, bridle roll units are generally provided after andbefore the quenching zone to maintain the tension of the steel strip atthe quenching zone at a high level. The rolls installed inside thesebridle roll units are preferably the rolls within the preferred rangesof the invention so as to avoid problems, such as slipping, buckling andmeandering, as described above. Under a high tension at the quenchingzone, meandering is likely to occur when rolls have concave-shapedcrowns, R<0, whereas buckling is likely to occur when rolls haveconvex-shaped crowns, R>0.

[0071] The optimum shape of the roll for avoiding these problems is aflat roll of R=0 located substantially in the center of the preferredrange shown in FIG. 9. In the flat roll, TR=∞ (radius of curvature=0).Other advantages of the flat roll are its ease of manufacturing and lowmanufacturing costs. In the bridle roll unit, the tension of the steelstrips is relatively low at the roll closest to the quenching zonecompared to the rolls at the preceding positions. In this respect, it ispreferable that, among these bridle rolls, the one closest to thequenching zone be a flat roll of R=0 and TR=∞ and the preceding rolls bethe rolls satisfying the conditions of the invention.

[0072] Preferably, the cooling gas injected into the quench zone isprevented from reaching inside of the bridle roll unit as much aspossible. When a large amount of cooling gas passes through theconnecting portion between the quenching zone and the bridle roll unitand reaches the bridle roll unit, the edges of the rolls inside the unitare excessively cooled, generating remarkable thermal crowns in thecentral portion of the roll. Thus, when the gauge of the strip isreduced, the probability of slipping and buckling becomes high.

[0073] In order to solve this problem, at least one pair of seal rollsis preferably installed in each of the connecting portions of the bridleroll units located at the entry side and the exit side of the quenchingzone.

[0074] Table 1 shows profiles of the rolls of the invention as installedinside the bridle roll units disposed before and after the quenchingzone of the upright continuous annealing furnaces. The profiles aredefined as Lc and R, and shows whether undesirable phenomena such asslipping, buckling, and meandering occur or not. In Table 1, “A”indicates neither slipping, buckling, nor meandering was observed; “B”indicates slipping, buckling, or meandering was occasionally observed;and “C” indicates slipping, buckling, or meandering was frequentlyobserved.

[0075] The quenching zone had a gas jet cooling unit having a capacityof 170 W/(m²·° C.) or more reduced to a heat transfer coefficient persteel strip surface and was provided with a pair of seal rolls at theentry of the quenching zone. The temperature range of 300° C. or morewas quenched. TABLE 1 Length of Inclination Curvature Width Flat R ofRadius TR of Portion Tapered at Strip Lc Portion Boundary Sample (mm)W_(min) W_(max) (mm) (×10⁻³) (m) Slipping Buckling Meandering Reference1 600 600 1600 700 0.2 20 A A A Example 2 1000 600 1600 700 0.2 20 A A AExample 3 1250 600 1600 700 0.2 20 A A A Example 4 1600 600 1600 700 0.220 A A A Example 5 600 600 1600 700 −0.05 20 A A A Example 6 1000 6001600 700 −0.05 20 A A A Example 7 1250 600 1600 700 −0.05 20 A A AExample 8 1600 600 1600 700 −0.05 20 A A A Example 9 1000 700 2000 10000.0 ∞ A A A Example 10 1250 700 2000 1000 0.0 ∞ A A A Example 11 1600700 2000 1000 0.0 ∞ A A A Example 12 1850 700 2000 1000 0.0 ∞ A A AExample 13 2000 700 2000 1000 0.0 ∞ A A A Example 14 1000 700 2000 800−0.2 20 B A A Comparative Example 1 15 1250 700 2000 800 −0.2 20 B A BComparative Example 1 16 1600 700 2000 800 −0.2 20 C A C ComparativeExample 1 17 1850 700 2000 800 −0.2 20 C A C Comparative Example 1 181000 700 2000 800 0.4 20 B B A Comparative Example 2 19 1250 700 2000800 0.4 20 C B A Comparative Example 2 20 1600 700 2000 800 0.4 20 C C AComparative Example 2 21 1850 700 2000 800 0.4 20 C C A ComparativeExample 2

[0076] As can be understood from Table 1, in samples 14 to 17(Comparative Example 1), the inclination R of the tapered portions ofeach of the rolls was negative; hence, significantly large concavecrowns were formed, resulting in slipping and buckling of the steelstrips.

[0077] In samples 18 to 21 (Comparative Example 2), the inclination R ofthe tapered portions of each of the rolls was positive; hence,significantly large convex crowns were formed, resulting in slipping andmeandering of the steel strips.

[0078] Samples 1 to 13 are rolls according to the invention.

EXAMPLES

[0079] Operations were conducted using an upright continuous annealingfurnace having a quenching zone unit including rolls of the inventiondisposed before and after a quenching zone.

[0080] The upright continuous annealing furnace had a W_(min) value of700 mm and a W_(max) value of 1850 mm. The quenching zone unit used wasthat shown in FIG. 1. The quenching zone included a gas jet coolingapparatus having a capacity of 170 W/(m²·° C.) or more, reduced to aheat transfer coefficient per steel strip surface.

[0081] An operation according to the invention satisfied all conditions(1) to (3). That is, in the operation of the invention, all of the rollsused inside the bridle roll units were in conformity with conditions (1)to (3). The operations, each satisfying only one of conditions (1) to(3), were also performed and were compared to a conventional process.

[0082] In the operation according to the invention satisfying all ofconditions (1) to (3), the rolls disposed before and after the quenchingzone had the following profiles: Lc=1.0×700, R=0.05×10⁻³, and TR=50 m.

[0083] In the operation satisfying only condition (1), the rollsdisposed before and after the quenching zone had the following profiles:Lc=1.0×700, R=0.4×10⁻³, and TR=10 m.

[0084] In the operation satisfying only condition (2), the rollsdisposed before and after the quenching zone had the following profiles:Lc=0.5×700, R=0.05×10⁻³, and TR=10 m.

[0085] In the operation satisfying only condition (3), the rollsdisposed before and after the quenching zone had the following profiles:Lc=0.5×700, R=0.4×10⁻³, and TR=50 m.

[0086] In the operation performed according to a conventional process,the rolls disposed before and after the quenching zone had the followingprofiles: Lc=0.5×700, R=0.5×10⁻³, and TR=8 m

[0087]FIG. 10 shows the relationship between each of the operationconditions and the decrease in yield of the products due to slipping,buckling, and meandering between the rolls and the steel strip. In thegraph, the yield reduction rate is normalized by the amount of thedefective product relative to the entire production in a conventionalprocess.

[0088] When slipping, meandering, or buckling occur, the speed of theline must be reduced, resulting in a reduced production yield.

[0089] Improvements compared to the conventional process can be attainedby satisfying one of conditions (1) to (3), but when all of theseconditions are satisfied in combination as in the invention, decrease inyield can be significantly improved to approximately one-tenth of theconventional process.

[0090]FIG. 11 shows the relationship between each of conditions (1) to(3) and operation efficiency reduction rate of the line caused byslipping, buckling, or meandering between the rolls and the steel strip.

[0091] Here, “operation efficiency” is defined as the ratio of the linespeed calculated from the capacity of the equipment to the actualoperation speed and is an indicator of the capacity in operation. In thegraph, the operational efficiency reduction rate is normalized by anaverage value of the difference between a theoretical line speedcalculated from capacity and an actual line speed of a conventionalprocess.

[0092] When slipping, buckling, or meandering occurs, the speed of theline is decreased, resulting in a reduced treatment speed. It may bepossible to continue the operation in such a state without majorproblems, but the operation efficiency will eventually be decreased,failing to achieve an expected production amount. If the problems aremajor, it becomes necessary to stop the line, decrease the temperatureof the furnace, and dispose of the steel strips in the furnace, thusfailing to achieve a predetermined production amount and decreasing theoperation efficiency.

[0093] By employing the rolls of the invention, the operation rate ofthe upright continuous annealing furnace was improved by 0.1% on averageand the operation efficiency reduction rate was lowered to one-fifthcompared to the conventional process.

[0094] Also, occurrence of line shutdown, decrease in the line speed,and so forth due to slipping, buckling, and meandering were maintainedat minimum levels, thereby significantly improving the production yieldand operation efficiency of the furnace.

What is claimed is:
 1. A roll for disposing at an entry side or an exitside of a quenching zone of a continuous annealing furnace, the rollsatisfying the following relationships: Lc≧0.7×W_(min); R=−0.1×10⁻³ to+0.2×10⁻³; and TR≧20 wherein Lc represents a length (mm) of a flatportion in the center of the roll, W_(min) represents a minimum width(mm) of a steel strip, R represents the inclination of tapered portionsdisposed at two sides of the roll, and TR represents the radius ofcurvature (m) of boundaries between the flat portion and the taperedportions.
 2. A quenching zone unit of a continuous annealing furnace,comprising: a quenching zone having an entry side and an exit side; andat least one roll selected from the group consisting of hearth rolls andbridle rolls disposed at at least one of the entry side and the exitside of the quenching zone, wherein the at least one roll comprises theroll according to claim
 1. 3. A quenching zone unit of a continuousannealing furnace, comprising: a quenching zone having an entry side andan exit side; and at least one bridle roll unit comprising a pluralityof rolls, the at least one bridle roll unit being disposed at at leastone of the entry side and the exit side of the quenching zone, whereineach of the plurality of rolls comprises the roll according to claim 1.4. The quenching zone unit of a continuous annealing furnace accordingto claim 3, wherein a roll of the plurality of rolls that is closest tothe quenching zone is a flat roll satisfying the relationships (i) R=0and (ii) TR=∞.
 5. The quenching zone unit of a continuous annealingfurnace according to claim 2, comprising at least one pair of seal rollsdisposed at at least one of the entry side and the exit side of thequenching zone.
 6. The quenching zone unit of a continuous annealingfurnace according to claim 3, comprising at least one pair of seal rollsdisposed at at least one of the entry side and the exit side of thequenching zone.
 7. The quenching zone unit of a continuous annealingfurnace according to claim 4, comprising at least one pair of seal rollsdisposed at at least one of the entry side and the exit side of thequenching zone.